If you find that you need to offer some non-standard date or time formats to the general settings page, this will help.
add_filter('date_formats', 'add_date_format');
function add_date_format($original) {
$more = array('D, M jS, Y');
return array_merge($original, $more);
}
add_filter('time_formats', 'add_time_format');
function add_time_format($original) {
$more = array('H:i:s a');
return array_merge($original, $more);
}